They began by promoting each other's books. Gingrich said Kerry's new book, "This Moment on Earth: Today's New Environmentalists and Their Vision for the Future," is "a very interesting read" and said he agrees with 60 percent of it. Kerry hasn't read Gingrich's new book on the environment, due out later this year, but said he has always enjoyed their exchanges.

Then the two argued for nearly two hours about whether the government should cap emissions of greenhouse gases or whether tax breaks will encourage businesses to do so.

Kerry said no environmental crisis has ever been solved voluntarily, without government intervention. "That's like saying, 'Barry Bonds, go investigate steroids,' or letting Enron take control of pensions," said the Massachusetts senator and 2004 Democratic presidential nominee.

Gingrich, the former House speaker, said a government cap amounts to more regulation and more lawsuits: "America changes much faster in the marketplace. We can move very rapidly if we're moving there as consumers."

The arguments were typical for their political parties, although Gingrich is farther to the left of some Republicans who dispute the science behind climate change.

"My message, I think, is that the evidence is sufficient that we should move toward the most effective possible steps to reduce carbon loading in the atmosphere," Gingrich replied.

He explained that conservatives often worry that the prescription for environmental problems will be bigger government and higher taxes.

The two debated in the historic caucus room of the Russell Senate Office Building, site of hearings on Watergate in 1973, the Iran Contra scandal in 1987 and the confirmation of Supreme Court Justice Clarence Thomas in 1991.

Hosted by New York University's John Brademas Center for the Study of Congress, the debate was more free-flowing than traditional presidential debates, with no time limits and very little interruption from the moderator.

Gingrich has advocated this type of unrestricted format for presidential campaigns. He's been considering his own campaign for the White House but says he won't decide until after September.